In order to obtain maximum speed in signal transmission on a wafer surface, it would be of interest to be able to use light to transmit signals over a given wafer, since light transmission is faster than transmission purely by way of an electric circuit, the difference being several orders of magnitude. As it was pointed out in the International Technology Roadmap for Semiconductors (ITRS), Interconnect Chapter (Semiconductor Industry Association, 2005, available at http://www.itrs.net), the RC (resistance times capacitance) delay time constant has become much larger than the transistor delay, thus dominating the circuit performance. In spite of the use of copper and low-k dielectric, the shrinking cross-section of the metal interconnects and their ever closer spacing on the chip significantly slow down signal propagation through the circuit. In the example given in the ITRS Interconnect chapter, 250 psec for 1 mm of a copper line could be compared to less than 10 psec for a signal transmitted optically. Another, even more important factor is that the bandwidth (i.e. the amount of information) that can be carried by an optical signal is usually much greater than what is possible with an electrical signal. And since optical fiber communication already dominates in telecom applications and is spreading to shorter distance links, such as between computers in a single building, it is important to build integrated optical circuits in which light signals are launched and processed before entering an optical fiber, or conversely light signals are received from an optical fiber, processed, and converted to electrical signals. An opto-electronic module, in which one part processes light before it is sent out and another part processes light received from outside is called a transceiver, as it has a transmitter and a receiver.
Optical circuits based on silicon-on-insulator (SOI) wafers exist, but light sources are external, not comprised within the SOI wafer. Individual components of optical circuits in silicon wafers, typically by taking advantage of SOI structures, have been demonstrated (Intel modulator, Luxtera transceiver, Ge photodetectors by selective epi on Si, SOI waveguides). In all these examples the light sources are external to the SOI wafer, either coupled by optical fibers or attached to the wafer surface. In one recent publication (A. Fang, R. Jones, H. Park, O. Cohen, O. Raday, M. Paniccia, and J. Bowers, “Integrated AlGaInAs-silicon evanescent race track laser and photodetector,” Opt. Express 15, 2315-2322 (2007), the laser sources are hybrid structures, with AlGaInAs light emitters coupled by bonding to SOI surface so that the laser cavity is partially in AlGaInAs and partially in Si below it.